The principal goal of this research project is the characterization of the dynamic state of biological membranes, with the objective of exploring the causal relationships between the molecular composition and organization of a membrane, its fluid dynamics, and specific membrane functions. A variety of modern optical methods are to be used, principally the recently developed fluorescence redistribution after photobleaching. Primary emphasis in the recent past has been placed on the study of relatively simple membrane systems. Measurements of the lateral diffusion coefficients of bacterial outermembrane components in vitro and in vivo have yielded new insights into the effects of steric interactions within the membrane, and the possible role that translational diffusion may play in the translocation process of membrane biogenesis. Studies on normal and spectrin-deficient spherocytic erythrocytes have demonstrated that the submembranous protein matrix in normal cells acts to decrease integral membrane diffusion rates by nearly two orders of magnitude. In the upcoming years, emphasis will shift back to the study of whole cells in culture, concentrating on the membrane related events associated with cell division, and the chemotaxis of neutrophilic polymorphonuclear leucocytes.